environment variable with su - and systemd-run su -

If the environment variable DISPLAY is set and I do `/bin/su - root` from a user shell in an X terminal, then a login shell for root is setup and the DISPLAY environment variable is still present. If I do `/usr/bin/systemd-run --quiet --setenv DISPLAY -t /bin/bash -c 'echo "DISPLAY is ${DISPLAY}" ; export DISPLAY=${DISPLAY} ; DISPLAY=${DISPLAY} /bin/su - root'` [Yes having both the export and the DISPLAY= before the command is probably overkill.] then when the login shell for root arrives, the DISPLAY environmental variable is not set, even though it was present and correctly echoed from the systemd-run process before the shell started up. (And I have tried doing --setenv DISPLAY=some_other_value just to check that a distinct value is being passed.) Testing the complicated `/bin/bash -c 'echo "DISPLAY is ${DISPLAY}" ; export DISPLAY=${DISPLAY} ; DISPLAY=${DISPLAY} /bin/su - root'` on its own just to check that starting the /bin/su -root from a bash command works, the display environmental variable is present in the root login shell. Omitting the bash invocation and just using /bin/su with the systemd-run command does not work and I include the bash invocation because of the answer to the earlier question systemd-run does not set environment variables when using --setenv So why is the DISPLAY not kept when the su login shell is started from systemd-run even though it has been correctly passed to the systemd-run environment with the setenv parameter? For reference, this is on openSUSE Leap 15.6 with systemd 254 (254.20+suse.113.gf7f6a3454e).